lib: Add API documentation

lib/error.php

@@ -1,12 +1,43 @@
 <?php
+/**
+ * This file is part of RSS-Bridge, a PHP project capable of generating RSS and
+ * Atom feeds for websites that don't have one.
+ *
+ * For the full license information, please view the UNLICENSE file distributed
+ * with this source code.
+ *
+ * @package	Core
+ * @license	http://unlicense.org/ UNLICENSE
+ * @link	https://github.com/rss-bridge/rss-bridge
+ */
+
+/**
+ * Throws an exception when called.
+ *
+ * @throws \HttpException when called
+ * @param string $message The error message
+ * @param int $code The HTTP error code
+ * @link https://en.wikipedia.org/wiki/List_of_HTTP_status_codes List of HTTP
+ * status codes
+ */
 function returnError($message, $code){
 	throw new \HttpException($message, $code);
 }
 
+/**
+ * Returns HTTP Error 400 (Bad Request) when called.
+ *
+ * @param string $message The error message
+ */
 function returnClientError($message){
 	returnError($message, 400);
 }
 
+/**
+ * Returns HTTP Error 500 (Internal Server Error) when called.
+ *
+ * @param string $message The error message
+ */
 function returnServerError($message){
 	returnError($message, 500);
 }